1. Field of the Invention
The present invention relates to an electroluminescent element. More specifically, the invention relates to an electroluminescent element having an electroluminescent layer, a part of which comprises host materials and guest materials.
2. Related Art
An electroluminescent element comprises an electroluminescent layer interposed between a pair of electrodes (anode and cathode). The emission mechanism is as follows. Upon applying voltage through a pair of electrodes, holes injected from an anode and electrons injected from a cathode are recombined with each other within the electroluminescent layer to lead the formation of molecular excitons, and the molecular excitons return to the ground state while radiating energy to emit photon. There are two excited states possible from organic compounds, a singlet state and a triplet state. It is considered that light emission can be obtained through both the singlet state and the triplet state.
Although an electroluminescent layer may have a single layer structure comprising only a light-emitting layer formed by a light-emitting material, the electroluminescent layer is  formed to have not only a single layer structure comprising only a light-emitting layer but also a lamination layer structure comprising a hole injecting layer, a hole transporting layer, a hole blocking layer, an electron transporting layer, an electron injecting layer, and the like which are formed by a plurality of functional material. Color tone can be appropriately changed by doping guest materials into host materials in the light-emitting layer. There is some possibility of improving luminance and lifetime of light emission in some combination of the host material and the guest material.
It has been disclosed that the quantum efficiency or the durability is improved of an electroluminescent element comprising host materials and guest materials, for example, tris(8-hydroxyquinoline)aluminum (also referred to as Alq3) as host materials and a coumarin derivative as guest materials. (For example, reference 1: Unexamined Patent Publication 2001-76876.)
In addition, it has been disclosed that an electroluminescent element comprising host materials and guest materials which is superior in luminous efficiency, durability, color purity characteristics, and the like to those of the conventional electroluminescent element can be provided by selecting host materials having the peak intensity of emission spectrum in a certain range. (For example, reference 2:  Unexamined Patent Publication 2001-118683.)
However, such electroluminescent element has been practically insufficient yet in terms of luminous efficiency or luminous characteristics. It is desired to develop an electroluminescent element having further superior device characteristics.